Laboratory fume hoods having adjustable baffle systems are well known as disclosed, for example, in co-owned U.S. Pat. Nos. 4,177,717, 4,434,711, 3,218,953, 3,217,630, 3,747,505, and 3,747,504. Typically, such a fume hood takes the form of a housing defining a chamber with an access and gas-inlet opening along the lower front portion of the hood. Baffles are arranged in a vertical series along the rear of the chamber, usually with the uppermost baffle or baffles sloping forwardly and, at least in some cases, substantially limiting work space within the upper portion of the chamber. Gases, as they are exhausted, are drawn through slots or openings between the successive baffle plates of the series, as well as about the side edges of those plates, and are carried upwardly behind the baffle plates into the exhaust opening. Other patents disclosing laboratory fume hood constructions are U.S. Pat. Nos. 3,604,333, 3,318,227, 3,111,077, 3,593,646, and 3,752,056.
It has been found that, in the operation of a typical laboratory fume hood of the type described, uncontrolled turbulence tends to develop in the upper portion of the hood chamber, and the extent of such turbulence varies with the positions of adjustment of the baffle plates, the extent of obstruction of the hood's front opening, and other factors. Such turbulence, and particularly the variability of such turbulence under different operating conditions, adversely affects uniformity of hood operation and the rate of evacuation of gases from the upper portion of the hood chamber.
The fume hood of this invention incorporates a baffle system that includes at least three baffle plates of generally rectangular configuration arranged in a stepped vertical series with their upper and lower edges spaced apart to define horizontal exhaust slots for the flow of gases from the chamber into the hood's exhaust opening. A pair of exhaust ports are provided at the top corners of the uppermost baffle plate to promote direction and control over the evacuation of gases circulating in the upper portion of the chamber. Specifically, air entering the hood through the inlet opening develops a vortical action that is given lateral direction as the spiraling gases travel towards the ports. Such gases continue through the ports and into the exhaust opening located centrally behind and above the upper baffle plate.
The upper and lower baffle plates of the series are fixed, but the intermediate plate may be either fixed or adjustable. If adjustable, the intermediate plate is mounted for pivotal movement about a horizontal line extending along its lower edge so that adjustment of the plate increases or decreases the size of the exhaust slot formed between the upper edge of the intermediate plate and the lower edge of the upper plate.
Unlike the upper baffle plate of a conventional hood, the fixed upper baffle plate of the hood disclosed herein is relatively vertical and provides greater space within the upper portion of the hood chamber. Gases are exhausted from the hood through a rectangular exhaust opening that has a forward-rearward dimension substantially the same as the distance between the upper baffle plate and the rear panel of the hood. A rectangular collar is externally mounted upon the hood and includes vertical front and rear walls that are aligned with the upper baffle plate and the hood's rear panel, respectively. The side walls of the collar curve downwardly and outwardly to merge with the hood's top panel, giving the hood a bellmouth configuration when viewed in transverse vertical section and promoting a smooth flow of gases through the rectangular exhaust opening. Because of its shape, the bellmouth collar may be installed by forming a rectangular notch in the top panel of the hood, thereby simplifying manufacture and installation, and avoiding potential problems of structural weakness that tend to be associated with the provision of circular exhaust openings.
Such a construction has been found to increase draw along the work surface and render it more uniform under various operating conditions. In general, fume capture within the hood is significantly enhanced and face velocities through the inlet or access opening of the hood may be maintained within a range of plus or minus 10% over the range of adjustment of the baffle system. Static pressure is also significantly reduced, resulting in appreciable savings in energy costs. Benefits from such a construction may also be realized even in those installations where the intermediate baffle plate is non-adjustable.
In a preferred embodiment, the intermediate baffle plate may be adjusted by an improved remote control mechanism. Such mechanism includes a knob-equipped control rod having multiple positions of adjustment that may be selected either visually (by color-coding for the different positions), or through tactile feedback, or both.
Other features, advantages, and objects will be apparent from the specification and drawings.